Fix fallback to global if extruder_nr doesn't exist

Fixes #6590.

plugins/CuraEngineBackend/StartSliceJob.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2018 Ultimaker B.V.
+# Copyright (c) 2019 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
 
 import numpy
@@ -72,7 +72,7 @@ class GcodeStartEndFormatter(Formatter):
         value = default_value_str
         # "-1" is global stack, and if the setting value exists in the global stack, use it as the fallback value.
         if key in kwargs["-1"]:
-            value = kwargs["-1"]
+            value = kwargs["-1"][key]
         if str(extruder_nr) in kwargs and key in kwargs[str(extruder_nr)]:
             value = kwargs[str(extruder_nr)][key]